This invention relates to transmission controls for a power transmission.
Power transmissions of the automatic shifting type have a plurality of selectively establishable gear ratios between the engine and ground engaging mechanism for the vehicle. The ratios are generally established by hydraulically operated friction devices such as clutches and brakes. The engagement and disengagement of these friction devices are controlled by valve mechanisms which direct hydraulic fluid to and from the operating piston of the devices. With the advent of electronically controlled transmissions, each valve mechanism is controlled by a separate electrical element such as a solenoid. In the event one of the valve-controlling solenoids becomes inoperative, it is desirable for the transmission control system to ensure that only allowed combinations of friction devices may be engaged simultaneously so not more than one drive ratio occurs.
The present invention provides a transmission control system for an automatic transmission, which ensures that simultaneous establishment of more than one drive ratio does not occur.
The control system comprises a manual selector valve for operator direction of line pressure fluid to a drive passage when drive is selected or to a reverse passage when reverse is selected. A drive/reverse passage connects the drive passage and the reverse passage. A first check ball is disposed between the drive passage and the drive/reverse passage and a second check ball is disposed between the reverse passage and the drive/reverse passage wherein the check balls operate to pressurize the drive/reverse passage when either drive or reverse is selected. The system further includes six fluid operated selectively engageable friction devices and three friction device actuators each associated with one of the six friction devices, in parallel relation and electronically actuated to direct fluid from the drive passage to the respective friction device. The system also includes a two-friction device actuator operative to direct fluid from the drive/reverse passage to one of two friction devices where a mode valve and associated actuator determine which of the two friction devices are pressurized. Three pressure-operated feed protection valves are disposed intermediate of two of the actuators and their two friction devices. Each feed protection valve may receive at least one signal pressure by open actuators feeding downstream friction devices. The signal pressures shift the valve to prevent flow to downstream friction devices which are incompatible to be engaged simultaneously.
The system minimizes the number of components while still ensuring the transmission is protected against any combination of actuator interruption.